The present invention relates to a simultaneous multi-beam optical modulation system, which can be employed in a laser printing apparatus.
In a well-known simultaneous multi-beam optical modulation system, a plurality of image signals are assigned to carriers, respectively and the amplitude of each carrier is modulated, whereby a plurality of modulated signals are produced and, at the same time, by the modulated signals, an acoustic optical element is actuated, so that a laser beam is divided and modulated by the acoustic optical element. This system is employed in a laser printing apparatus and is useful in lowering the deflection speed of a scanning optical deflection apparatus. In this system, however, since the acoustic optical element is actuated simultaneously by a plurality of modulated signals, the light modulation intensity of each image signal is changed under the influence of the other image signals, so that cross modulation occurs between the multiple beams from the acoustic optical element.
A system, as shown in FIG. 1, has been proposed for the purpose of obviating such cross modulation. In this system, a plurality of image signals f.sub.s1 -f.sub.sm from a signal source (not shown) respectively modulate the amplitude of the carriers assigned to the respective image signals, from high frequency oscillators OSC.sub.1 -OSC.sub.m in AM modulators M.sub.1 -M.sub.m. An addition-reduction inverting amplifier OP comprising resistors R.sub.0 to R.sub.n (where R.sub.0 =R.sub.1 =R.sub.2 =R.sub.3 =. . . R.sub.n), a diode D.sub.0 and a direct current amplifier A, performs addition of the image signal input voltages f.sub.s1 to f.sub.sm and reduction of a setting voltage f.sub.sn, set by a variable resistor VR.sub.0 and also performs inverse amplification of the image signal input voltages f.sub.s1 to f.sub.sm and the setting voltage f.sub.sn'. The setting voltage f.sub.sn, is a voltage for setting the overall deflection efficiency of an acoustic optical element AO and is produced in a setting voltage means or setting means by dividing a power source whose polarity is opposite to that of the image signal input voltages f.sub.s1 to f.sub.sm by use of a variable resistor VRO. In other words, the addition-reduction inverting amplifier OP produces a dummy signal f.sub.sn by addition of the image signal input voltages f.sub.s1 to f.sub.sm and the setting voltages -f.sub.sn' , followed by inversion thereof, namely f.sub.sn =-{(f.sub.s1 +f.sub.s2 +. . . +f.sub.sm)-f.sub.sn' }. The thus-produced dummy signal f.sub.sn modulates the amplitude of carrier assigned by a high frequency oscillator OSC.sub.n in an AM modulator M.sub.n.
The output signals of the modulators M.sub.1 to M.sub.n are summed up into one signal by a mixer MIX through gain control amplifiers GC.sub.1 -GC.sub.n. The summed signal is amplified by a power amplifier PA and is then applied to the acoustic optical element AO. The acoustic optical element AO divides and modulates a laser beam I.sub.in from a laser generating apparatus (not shown) and produces deflected light beams of first order, I.sub.11 to I.sub.1m and I.sub.1n which respectively correspond to the image signals f.sub.s1 to f.sub.sm and the dummy signal f.sub.sn and a light beam of zero order I.sub.0. When the dynamic ranges of the image signals f.sub.s1 to f.sub.sm are made equal, the characteristics of image signal to first order deflected light are scattered with respect to each beam. This scattering is caused by the employed electric circuits, the frequency band of the acoustic optical element AO and across modulation between the beams. Since the scattering of the characteristics of image signal to the first order deflected light is caused by the difference of the gain or the gradient of the input and output characteristics, the gain control amplifiers GC.sub.1 to GC.sub.m are adjusted so that the input and output characteristics of the respective beams are in agreement.
The minimizing of cross modulation between beams in a simultaneous multi-beam optical modulation system can be attained by use of an acoustic optical element having a broad frequency band and by keeping the intensity of the overall deflection of first order light constant, irrespective of the input of the image signals, and when the employed frequency band is broad, since the characteristics of image signal input to deflection of light of the first order do not differ so much in each beam, a system capable of reducing cross modulation can be realized by keeping the sum of the input of image signals constant. In the simultaneous multi-beam optical modulation system in FIG. 1, the sum of output beams I.sub.11 to I.sub.1m and a dummy beam I.sub.1n, namely .SIGMA.I.sub.1 =I.sub.11 +I.sub.12 +. . . I.sub.1m +I.sub.1n is kept constant. In order to accomplish this, a dummy signal voltage f.sub.sn is produced in a manner to make the sum of image signal input voltages f.sub.s1 to f.sub.sm and dummy signal voltage f.sub.sn, namely .SIGMA.f.sub.s3 =f.sub.s1 +f.sub.s2 +f.sub.sm. . . +f.sub.sn, constant.
The simultaneous multi-beam optical modulation system in FIG. 1 is premised on I.sub.11 .alpha.f.sub.s1, I.sub.2 .alpha.f.sub.s2, . . . However, there is the following relationship between I.sub.11 and f.sub.s1 in practice. EQU I.sub.11 =I.sub.in sin.sup.2 Kf.sub.s1
where K is a constant. Therefore, this system does not have a sufficient cross modulation compensation effect.